1. Field of the Disclosure
The present disclosure relates to an air conditioning system. In particular, the present disclosure relates to an air conditioning system that performs at low ambient temperatures.
2. Description of the Related Art
Air conditioning systems are generally operated to condition (e.g., heat, cool, and/or dehumidify) air in a predetermined space such as, but not limited to, a house, a building, a car, a refrigerator, a freezer, and others.
Many air conditioning systems are not designed to operate in the cooling mode when the outdoor temperature is below a certain temperature, such as about 55° Fahrenheit. In general, as the air conditioner operates in the cooling mode below this temperature, the temperature and pressure of the refrigerant flowing through its outdoor heat exchanger coil falls below desirable levels. Performance variables of the air conditioning system, such as the cooling capacity and the efficiency, start dropping off. The reliability of the air conditioning system can also decrease as problems, such as liquid refrigerant flooding the compressor or the indoor heat exchanger coil freezing, can occur.
However, many applications of air conditioning systems are installed in settings where there may be a need for cooling even when the weather outside is below the predetermined set point. An example of such an application is a computer room inside a commercial building, where the equipment generates a lot of heat and yet requires moderately cool temperatures to operate properly and reliably. Thus, there is a desire for air conditioning systems that are configured to maintain efficient operation in a cooling mode at ambient outside temperatures below the predetermined set point.
In order to extend the operating range of a residential air conditioning system to lower outdoor temperatures, several low ambient cooling kits are currently available. While there are many variations of these kits, there are two main exemplary embodiments. The general object of these kits is to maintain either the temperature or the pressure of the air conditioning refrigerant fluid in the outdoor heat exchanger coil of the air conditioner within desirable limits, by either cycling or varying the speed of the outdoor fan motor that blows air over the outdoor heat exchanger coil.
One embodiment of these kits comprises a temperature sensor and electronic circuits used to vary the speed of a standard AC fan motor, which blows ambient air over the outdoor heat exchanger coil. The temperature sensor is installed at an appropriate location on the coil to sense liquid refrigerant temperature. The electronic circuit typically “chops” or cuts off parts of the AC sinusoidal voltage applied to the fan motor to vary its speed in relationship with the sensed refrigerant temperature. The fan motor speed is reduced more and more as the sensed temperature falls. The slower fan speed reduces the quantity of heat lost by the refrigerant in the coil to the outdoor air. If the refrigerant temperature increases, the fan speeds up, causing a greater rate of heat exchange from the refrigerant to the ambient air. This overall mechanism enables the regulation of refrigerant temperature within a relatively narrow desirable range over a wide range of outdoor air temperatures.
This kit, however, is expensive and requires a ball-bearing fan motor that can operate reliably over a wide speed range. Also, the electronic “chopping” method of varying the speed of the motor causes undesirable harmonics in the AC power line.
In another embodiment of the low ambient kits of the prior art, a pressure switch is installed at the appropriate location in the outdoor heat exchanger coil. This pressure switch turns on and off at predetermined refrigerant pressure levels. It is electrically connected in series with the outdoor fan motor. When the refrigerant pressure falls below one predetermined pressure level, the pressure switch turns the fan motor off. When the refrigerant pressure increases above a second predetermined level, the switch turns the fan motor back on. In this manner the fan motor is cycled to regulate refrigerant pressure.
This embodiment, while simpler, requires the installer to break into the refrigerant system to install the pressure switch, which may cause refrigerant leaks. It also typically results in rapid fan cycling, causing stresses on various components of the system. Furthermore, cooling capacity and efficiency drop off significantly at lower outdoor temperatures.
Both of these embodiments require several additional items to be installed in the air conditioning system. Air conditioning systems usually include a low pressure switch (LPS), which acts as a protection device and shuts the system down when the refrigerant pressure falls to a very low level, which usually indicates a loss of refrigerant. However, the LPS may falsely trip, even when refrigerant levels are normal, and cause nuisance shutdowns in the beginning of low ambient cooling cycles. For this reason, the low ambient kits require an LPS bypass timer switch, which bypasses the LPS, allowing continued operation, for a fixed period, typically 3 minutes, at the beginning of each cycle. After this fixed period, refrigerant pressures normally build up to levels that reset the LPS and the bypass is not needed. If the LPS remains tripped beyond this fixed period, this clearly indicates a system fault and the consequent shutdown is justified.
Low ambient cooling operation can also result in the freezing of the indoor heat exchanger coil in the air conditioning system. For this reason a freeze thermostat is required to be installed on the indoor coil. This is simply a temperature switch that senses a freezing temperature on the coil and shuts the air conditioner down.
Recently, dual stage air conditioning systems are becoming more popular. In such systems, the first stage delivers a significantly lower cooling capacity by reducing the refrigerant flow rate produced by the compressor, while the second stage delivers the full cooling capacity. The first stage is typically significantly more energy efficient, while its lower capacity is sufficient to match the cooling demand during milder weather conditions. The less efficient second stage is available to deal with the higher demand when the weather is hotter.
It is much more difficult to apply the prior art kits to the newer and more energy efficient dual stage air conditioners. This is because the two stages have two different refrigerant flow rates, and therefore two different desired refrigerant temperature and/or pressure ranges, one for each stage. The above kits can be optimized for only one such range, for one of the stages, with performance or reliability degradation occurring in the other stage. It is also complex and impractical to “double up” the kits and install to of them on a dual stage air conditioning system. For this reason, the prior art low ambient cooling kits have not been found acceptable for dual stage systems.
Accordingly, there is a continuing need for air conditioning systems that can cool efficiently at low ambient temperatures and overcome one or more of the aforementioned and other disadvantages of currently available air conditioning systems.